Flow rate control mechanisms are used in a variety of flow systems as a means for controlling the amount of fluid, gaseous or liquid, traveling through the system. The flow control mechanisms can be utilized to regulate flow rates in systems such as ventilators and respirators where, for example, it may be desirable to maintain a sufficient flow of breathable air or provide sufficient anesthetizing gas to a patient in preparation for surgery.
MEMS based flow sensors can be utilized for measuring such flow rates in a variety of commercial, industrial and medical applications. In medical applications, for example, it is often required to accurately measure the flow rates of fluids introduced intravenously to medical patients and thereby control the flow rate of such fluids. In such applications, flow control is an inherent aspect of proper operation, which can be achieved in part by utilizing the flow sensors to measure the flow rate of fluid within the flow system.
Ventilators are medical devices for delivering a breathing gas to a patient. Usually, ventilators employed in hospital critical care units provide a supply of air enriched with oxygen for inspiration by the patient, and may conventionally include controls for either assisting or controlling breathing, exhaled volume indicators, alarms systems, positive end expiratory pressure valves, pressure indicators, gas concentration monitors, flow indicators, and heated humidifiers for warming and humidifying the breathing gas. Ventilators used in home care are often used to treat obstructive sleep apnea and supply positive air pressure to assist breathing. Manufacturers of medical ventilator equipment require an ultra low pressure drop to insure efficient blower operations.
The majority of prior art airflow sensors utilized for medical ventilators operate on a principle of a flow restrictor, traversing the air stream and measuring the pressure at a number of locations in the duct. The static pressure drives a sample of airflow through a bypass channel where the flow rate is measured.
An alternate technology uses a pitot tube having a probe with an open tip, which is inserted, into the flow field in order to measure a static pressure. The static pressure is an increasing, continuous function of the airflow rate within the tube. The pitot tube extends completely through the main channel of the sensor therefore presents a barrier to the oncoming flow. The problem associated with these sensors is that the sensor itself is responsible for a certain amount of turbulence in the flow channel. Sensor-generated turbulence causes an increase in pressure drop across the sensor as well as noise in the duct system.
Based on the foregoing it is believed that a need exists for an improved airflow sensor that reduces pressure drop and that is adapted to reduce obstruction to the flow. It is believed that the improved flow sensor disclosed herein can address these and other continuing needs.